As ceremonies began in Tiananmen Square to mark the one-year countdown to the Games, Jacques Rogge, president of the Olympic Committee, said concerns about the city's smog meant they were preparing contingency plans for next year.

"Yes, this is an option," Mr Rogge said when asked if postponement was a possibility.

"It would not be necessary for all sports, sports with short durations would not be a problem.

"But definitely the endurance sports like the cycling race where you have to compete for six hours, these are examples of competitions that might be postponed or delayed to another day."

Mr Rogge's comments are another sign of the worries the city's air quality, the worst of any of the world's major cities, is causing among organisers and athletes.

British, Australian and Canadian Olympic teams are among those who have said they will avoid the city in advance of the Games, acclimatising to Asia's heat elsewhere.

Simon Clegg, chief executive of the British Olympic Association, said its decision to use Macau for pre-Games training had been made partly with the pollution in mind.

"We are keenly aware of the environmental challenge," he told The Daily Telegraph. Mr Rogge was in Beijing for festivities, hosted by senior city and state leaders.

They were intended to show off some of the talents that will be on display at next year's opening ceremony, ranging from patriotic song and dance routines to syrupy hymns on the theme of friendship and harmony.

They opened with the song recently chosen as Beijing's anthem for the Games, "We're Ready".

Speeches followed from politburo members, including Wu Bangguo, China's number two ranking leader, who used Communist Party theory to praise Beijing's progress to Olympian stature.

Beforehand, in an interview with CNN, Mr Rogge said there would be no compromise on athletes' health, and that just as rain caused delays at Wimbledon, low air quality would mean long distance events were put off by a day or more.

IOC officials believe pollution is the major problem the Games currently face.

Neither the IOC nor the organisers mentioned concerns over human rights, despite attacks from groups ranging from Amnesty International to pro-Tibet activists.

Two more "Free Tibet" activists have been arrested in Beijing, according to supporters, bringing the number in detention to eight.

One of the two was a Briton, Paul Golding, as was one of six arrested after abseiling with a protest poster from the Great Wall.

China's demands that such groups not be allowed to "politicise" the Olympics won support from an aide to the mayor of London, Ken Livingstone.

Jude Woodward, his cultural adviser, was asked by a Hong Kong-based television station whether she opposed campaigners using the Games to highlight issues such as China's support for the regime in Sudan.

"I don't think it's the right thing to do," she said.

"The Olympic Games put the country (holding them) under the spotlight," she added.

"These people who disagree with some aspect of policy are taking advantage of that publicity for their own issues."